Rockwell Automation 8520-MUM 9/Series CNC Mill Operation and Programming Manual Documentation Set - Manual (Page 309)

Introduction to Programming
Chapter 10
10-35
End of Subprogram or Main Program Auto Start (M99)
M99 End of Subprogram or Paramacro program
When M99 is executed, subprogram execution is completed and
program execution returns to the calling program. This word is not
valid in an MDI command though it may be contained in a subprogram
called by an MDI command. For details on programming an M99, refer
to section 10.3.
M99 End of Main Program with Auto Start
If executing a program from memory, an M99 as the last block in a main
program causes program execution to stop at that location. The
program is reset to the first block and a <CYCLE START>
automatically starts program execution for you.
If executing a program from an external device (such as a tape reader),
when M99 is executed, program execution is stopped and the tape is
automatically rewound to the beginning of the program just executed
and a <CYCLE START> automatically starts program execution for
you.
CAUTION: The M99 code is commonly used as the end of
program for fully automated systems that automatically load the
next part to be machined. Typically this code requires that some
PAL interface be written that assures the part is fully loaded and
ready for machining before block execution is allowed to
restart. Failure to do so may cause injury to operators or
damage to equipment.
For these systems some PAL interface should be written to assure that
the part is fully loaded before program execution is restarted.
Simple Synchronization (M100-M149)
M100 - M149 — Simple Synchronization (dual-process system only)
These M-codes are for simple synchronization. When executed, this set
of M-codes does not re-setup any program blocks that have already been
read into program lookahead. See page 30-7.